1. Field of the Invention
The present invention relates to an inertial sensor and, more particularly, to an inertial sensor such as an acceleration sensor or gyro that possesses superior impact resistance.
2. Description of the Related Art
The inertial sensors such as acceleration sensors or gyros of recent years allow miniaturization, high performance and lower costs to be implemented simultaneously due to the development of microprocessing technology that applies MEMS (Micro Electro Mechanical System) technology. In this context, the installation of MEMS device inertial sensors in all kinds of devices requiring motion sensing is to be expected, such as devices directed toward car navigation, in-vehicle air bag control, the avoidance of image blurring due to hand movement for cameras, video cameras, and so forth, cellular phones, robot posture control, game gesture input recognition, HDD rotation and shock sensing, and so forth.
Further, such devices, for which motion sensing is required, can, on occasions, be subject to large unexpected shocks, and there is therefore the danger that large shocks will be applied to inertial sensors that are mounted in such devices. For example, in an automobile application such as an in-vehicle air bag, large shocks, which cannot occur during normal operation, can be applied to an inertial sensor in the event of a rear-end collision or vehicle roll and, in the case of a cellular phone or similar, shocks are to be expected when the cellular phone is dropped by mistake. Further, in the case of a hobby application such as a gaming device, extremely large shocks can occur when the gaming device is handled roughly by the user. Such accidental shocks, which are applied unexpectedly, may be said to be 3000 G and 5000 G, and therefore inertial sensors that are mounted in such devices require high impact resistance.
However, the inertial sensor has a basic constitution in which a movable weight portion is suspended by a beam. Therefore, known problems include the fact that when a large shock is accidentally applied, the beam is greatly deformed and damaged and the sensor function is lost. An invention for resolving this problem is disclosed by Japanese Patent Application Laid Open No. 2000-187041.
The capacitive-type acceleration sensor disclosed by Japanese Patent Application Laid Open No. 2000-187041 is such that buffer material is provided on an insulating protective cover that is housed with the moving part interposed therebetween as a device for preventing damage to the moving part due to shocks even in the event that the moving part and insulating protective cover should collide upon receiving a large shock. Further, the provision of this buffer material makes it possible to obtain an acceleration sensor with which there is no risk of measurement of the acceleration no longer being possible due to damage to the moving part as a result of dropping the device in which the sensor is mounted.
However, the acceleration sensor disclosed by Japanese Patent Application Laid Open No. 2000-187041 is an invention that was conceived in order to resolve the problem of carrying an acceleration sensor with a constitution in which 'a mass 4a is movably supported in parallel with a face joining glass substrates 6 and 7 and a silicon substrate 1 via a beam 4c (See Japanese Patent Application Laid Open No. 2000-187041, paragraph No. 0005). The acceleration sensor also has a constitution 'that implements a structure in which the width of the beam supporting the mass 4Aa is narrow in comparison with the thickness and it is difficult to displace the mass 4Aa toward the glass substrates 6A and 7A′ (See Japanese Patent Application Laid Open No. 2000-187041, paragraph No. 0026), the space within which the moving part can move being limited to a mostly two-dimensional space that is parallel to the face joining the glass substrates 6, 7 and the silicon substrate 1.
Further, an acceleration sensor is obtained with which there is no risk of measurement of the acceleration no longer being possible due to damage to the moving part as a result of dropping the device in which the acceleration sensor is mounted because a recess 6Aa that is approximately 15 μm deep is formed on the glass substrate 6A in the face opposite a protrusion 4Ab of the mass 4Aa and an aluminum layer 8 that is approximately 5 μm thick is deposited by means of vapor deposition as a buffer material for buffering shocks caused by collisions with the protrusion 4Ab on the bottom face of the recess 6Aa (See Japanese Patent Application Laid Open No. 2000-187041, paragraph No. 0027).
That is, the buffer material with which the acceleration sensor disclosed by Japanese Patent Application Laid Open No. 2000-187041 is provided is provided on the surface of a glass substrate on which the moving part is not displaced during normal operation of the acceleration sensor. Hence, there is the problem that, because the moving part moves in three dimensions during normal operation, this buffer material cannot be applied to an inertial sensor with a constitution that affords the original functions.
Therefore, the present inventors first proposed a constitution that allows the impact resistance to be improved also for an inertial sensor such as an acceleration sensor or gyro that comprises a moving part capable of three-dimensional movement (Japanese Patent Application No. 2004-099161).
The constitution proposed in the above application was such that, in an inertial sensor that comprises a sensing portion in which a weight portion, which is a moving part, is supported by a beam, a weight stopper portion is disposed close to the weight portion with a predetermined clearance from the weight portion by MEMS-processing part of the substrate of the sensing portion.
However, because this constitution is a structure in which a stopper portion for displacement of the weight portion in one direction is formed by MEMS-processing part of the substrate of the sensing portion, MEMS processing is complicated and it is necessary to either thicken the sensing portion substrate or narrow the weight portion, which is not advantageous from the perspective of reducing the height of the element or from the perspective of the detection sensitivity.